A circuit for storing halftone screen arrays which can be clocked and arranged so that a number of dots can be generated from a single Holladay brick of threshold values, the arrangement allowing a change of phase, shift and counting direction between scans.
The simplest halftone screen can be thought of as a set of threshold values stored in a memory of a printer. As a stream of multi-bit pixels is received, each is compared to its associated value in the screen, and printed either as a ONE or a ZERO. A number of types (single centered, multi-centered and stochastic) and sizes of halftone screens are required to optimally print various kinds of graphics, pictures and text. Larger screens can provide a greater number of color levels, at the expense of needing more storage. Also, in a four (or more) color printer, each separation is normally oriented at a different angle, and therefore must have its own screen. The storage of the resultant number of screens becomes a problem, and a means for reducing the memory requirement would be a significant improvement.
U.S. Pat. No. 4,185,304, Electronic Halftone Screening by Thomas M. Holladay, describes an efficient method of storing and using a screen which requires the minimum number of locations for each halftone dot, and is incorporated by reference herein. It was determined that if the minimum number of values were arranged into rows and columns, the result is a rectangle, or "brick", of numbers, each row of which of which is easily accessible to form each succeeding scan line of a halftoned image.
A large improvement would be to reduce the memory requirement by modifying the system so that one brick could be used to generate more than one halftone screen.